This invention relates to circuit interrupters and more particularly to current limiting circuit interrupters.
It is common in the prior art to provide for current limiting during circuit fault conditions. A common method of providing current limiting is through the use of current fuses used in combination with a standard circuit interrupter, similar to that shown in U.S. Pat. No. 3,077,525 to Dyer. A further method of providing current limiting capability is disclosed in U.S. Pat. No. 3,815,059 to L. A. Spoelman which teaches a circuit interrupter in series with an electromechanical current limiting device which utilizes a force generated by the overload current to drive the movable contact arm. In U.S. Pat. No. 3,077,525 the current limiting interrupter is provided with a movable contact arm which is pivoted around one end and which moves into the slot of a magnetic drive device during interruption. Another example of a current limiting device which utilizes a magnetic drive or slot motor to rapidly separate the contacts during predetermined overload conditions is found in U.S. Pat. No. 4,077,025 to Slade et. al.
Often these current limiting devices are utilized in electric circuits to protect electrical equipment such as motor starters which are not manufactured to withstand the surge currents which arise during severe overload conditions. Typically, motor starters are equipped with thermomagnetic type trip units, utilized for circuit breaking during nominal overload conditions, which when placed in series with typical interrupters provide enough resistance to the circuit to throttle the interrupt let through current to a level safe for the motor starter mechanisms. While typical circuit interrupters adequately protect motor starters utilizing the thermomagnetic tripping device from damage resulting from interrupt let through current, advances in the motor controller art have caused the substitution of the thermomagnetic tripping device with solid state tripping devices which the typical circuit interrupters are unable to protect. During severe fault conditions, the absence of the resistance supplied to the circuit from the use of the thermomagnetic trip devices in the new motor controllers results in a potentially higher let through fault current which the motor starter cannot tolerate, thus resulting in motor starter mechanism damage.
Inasmuch as the fault current flowing through connected electrical equipment such as motor starters will damage the equipment unless the fault current is limited, it is desirable to have and employ a circuit interrupter with current throttling capability. It is further desirable to provide the current throttling capability without drastically altering the typical interrupter structure to provide current throttling without added manufacturing expense. Since circuit interrupter manufacturing is normally conducted on a high volume basis, cost and simplicity is an important advantage. Likewise, the ability to rapidly reset and reuse the current limiting device is desirable, since resettable circuit interrupters have many well recognized advantages over fused devices.